The invention relates to an electric lamp comprising a glass lamp vessel sealed in a vacuum-tight manner and having an SiO.sub.2 content of at least 95% by weight, current supply conductors extending through the wall of the lamp vessel to an electric element arranged in the lamp vessel and the lamp vessel being coated with an interference filter of alternative layers of mainly SiO.sub.2 and of mainly metal oxide.
Such a lamp is known from British Patent Specification No. 2,103,830.
Due to the presence of an interference filter, the spectrum of the radiation emitted by the lamp is different from that in the absence of such a filter. The filter can be used in an incandescent lamp for reflecting infrared radiation so that thermal losses in the lamp are reduced and the lamp has a higher efficiency. Another possibility consists in having the filter reflect light of a given wavelength, as a result of which the lamp emits coloured light. The filter may also be used, for example, in metal halide discharge lamps, for example, for reflecting infrared radiation.
Incandescent lamps provided with an interference filter have long been known. For example, U.S. Pat. No. 4,017,758 shows and describes an incandescent lamp comprising a quartz glass lamp vessel which is enclosed by an outer bulb, while an interference filter is situated in the protective space between the lamp vessel and the outer bulb. However, the presence of an outer bulb is disadvantageous because it causes the generated light to be additionally reflected, as a result of which it can less readily be concentrated. Another disadvantage is that the outer bulb causes the lamp to become more voluminous, as a result of which luminaires of the lamp are more voluminous and the generated light can also be less readily concentrated.
The lamp disclosed in the aforementioned British Patent Specification No. 2,103,830 is an incandescent lamp which affords the advantage that the interference filter is present directly on the surface of the lamp vessel. Thus, an outer bulb as a carrier of the filter can be omitted. The interference filter consists of alternating layers of SiO.sub.2 and of Ta.sub.2 O.sub.5.
Silicon dioxide has a low refractive index and a high chemical and physical stability. This is evident from the use of glasses having a content of SiO.sub.2 of 95% by weight or higher for the lamp vessel of halogen incandescent lamps and for discharge lamps. Tantalum pentoxide has a comparatively high refractive index (n=2.13). A disadvantage of Ta.sub.2 O.sub.5, however, is its limited physical and chemical stability. Already after a thermal treatment at 800.degree. C. for about 30 minutes, Ta.sub.2 O.sub.5 in the filter has crystallizes to polycrystalline .beta.-Ta.sub.2 O.sub.5. This results in that the filter both scatters the transmitted light and reduces its capability to be concentrated and scatters the reflected radiation. If the filter is a filter reflecting infrared radiation, in an incandescent lamp less infrared radiation returns to the filament due to the diffuse reflection. Thus, this filter provides a smaller improvement in efficiency than would be the case without crystallization of the Ta.sub.2 O.sub.5 layers.
Tantalum pentoxide further has the disadvantage that during operation of the lamp, in which it is used as a filter component, it produces mechanical stresses which leads to the formation of cracks in the filter. These cracks are visible as craquele.
Another disadvantage is that Ta.sub.2 O.sub.5 becomes grey at high temperatures, which leads to a reduced transparency. In the absence of oxygen, as in the outer bulb of a discharge lamp, greying occurs to a greater extent than in air. However, in a discharge lamp having an outer bulb it may also be of importance that an interference filter is directly situated on the lamp vessel (the discharge vessel) because radiation reflected by the filter is less frequently reflected before it returns to the discharge.